Lyotropic liquid crystal nanofiltration membranes

ABSTRACT

The invention provides composite nanofiltration membranes (FIG.  5 ) with lyotropic liquid crystal (LLC) polymer porous membranes ( 30 ) attached to a porous support ( 20 ). The LLC membranes are prepared from LLC monomers which form the inverted hexagonal or bicontinuous cubic phase. The arrangement, size, and chemical properties of the pores can be tailored on the molecular level. The composite membrane of the invention is useful for separation processes involving aqueous and nonaqueous solutions as well as gases. Methods for making and using the composite nanofiltration membranes of the inventions are also provided.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is the National Stage of International Application No.PCT/US2003/031429, filed Oct. 3, 2003, which claims the benefit of U.S.Provisional Application No. 60/416,077, filed Oct. 3, 2002.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

This application was made at least in part with government support underOffice of Naval Research Grant N00014-02-1-0383. The United Statesgovernment has certain rights in this invention.

BACKGROUND OF THE INVENTION

This invention is in the field of composite nanofiltration membranes, inparticular composite nanofiltration membranes employing at least onelyotropic liquid crystal polymer porous membrane on a porous support.

Polymer membranes based on lyotropic liquid crystal (LLC) mesogens areof interest because of the ability of LLC mesogens to self-assemble intoordered, nanoporous aggregate structures in the presence of a solventsuch as water. The aggregates can be relatively highly ordered yet fluidcondensed assemblies with specific nanometer-scale geometries, knowncollectively as LLC phases (Gin et. al., “Polymerized Lyotropic LiquidCrystal Assemblies for Materials Applications,” 2001, Acc. Chem. Rec.24, 973-980). LLC mesogens are amphiphilic molecules containing one ormore hydrophobic organic tails and a hydrophilic headgroup. Surfactantscan be classified as amphiphiles (D. Considine, ed., Van Nostrand'sScientific Encyclopedia, Seventh Edition, 1989, Van Nostrand Reinhold,New York, p. 861).

Polymer membranes based on LLC mesogens have been reported. Beginn etal. reported membranes containing ion-selective matrix-fixedsupramolecular channels (Beginn, U.; Zipp, G.; Möller, M. “FunctionalMembranes Containing Ion-Selective Matrix Fixed SupramolecularChannels,” Adv. Mater. 2000, 12, 510). Solutions of2-hydroxymethyl-[1,4,7,10,13-pentaoxacyclopentadecane]-3,4,5-tris[4-(11-methacryloylundecyl-1-oxy)benzyloxy]benzoate,a tris-methacrylated crown ether amphiphile, in a mixture of monomers,cross-linkers, and a photo-initiator were reportedly cast to thin filmson a supporting porous filter (Pall Filtron NOVA membrane with maximumpore size of 10 microns). The mixture was subsequently cooled to −50° C.on a temperature-controlled aluminum block and then polymerized. Thecross-section of the supported membrane reportedly showed that thesupport was completely filled with the cross-linked methacrylate. Thesupramolecular channels were reportedly formed by self-assembly of thetris-methacrylated crown ether amphiphile into long cylindricalaggregates with the crown ether moieties stacked parallel to the columnaxis and the polymerizable groups forming the shell of the cylinder.

Beginn et al. also reported membranes containing oriented supramoleculartransport channels (Beginn, U.; Zipp, G.; Mourran, A., Walther, P., andMöller, M. “Membranes Containing Oriented Supramolecular TransportChannels,” Adv. Mater. 2000, 12, 513-516.). The membranes weresynthesized by filing the 400 nm wide pores of a track-etched polyestermembrane with a hot isotropic methacrylate solution of2-hydroxymethyl-[1,4,7,10,13-pentaoxacyclopentadecane]-3,4,5-tris[4-(11-methacryloylundecyl-1-oxy)benzyloxy]benzoate,a tris-methacrylated crown ether amphiphile (60 wt.-%). The filledpolyester membrane was cooled below the isotropization temperature ofthe lyotropic solution and the solution polymerized.

WO 98/30318 to Gin et al. states that polymer membranes can be formedfrom amphiphilic LLC monomers that will self-organize into stable,inverse hexagonal phases in the presence of pure water or otherhydrophilic solutions. It was further stated that in situphotopolymerization of the hydrophobic tails into a heavily cross-linkednetwork with retention of the template microstructure yields a robustpolymer network with highly uniform pores arranged in a regularhexagonal array. Formation of a polymer film between two glass slides byphotopolymerization of a LLC monomer mixture was reported. It wasfurther reported that the film could be peeled off the glass slides inone piece.

U.S. Pat. No. 5,238,613 to Anderson reports polymeric membrane materialshaving a pore size between two nanometers and sixty microns. Theporosity of the membrane materials is reported to be greater than fiftypercent. U.S. Pat. No. 5,238,613 states that binary water/polymerizablesurfactant bicontinuous cubic LLC phases could provide a route formembrane formation.

A need continues to exist for polymer membrane manufacturingtechnologies which allows control of critical structural features suchas pore size, pore architecture, and pore density in the nanometer sizeregime. A need also exists for polymer membranes for which thesecritical structural features can be controlled on this extremelyimportant size scale.

SUMMARY OF THE INVENTION

In an embodiment, the invention provides a composite nanofiltrationmembrane comprising: a porous support; and a LLC polymer porous membraneattached to the support. In an embodiment, the porous support isultraporous. In an embodiment, the pore size of the LLC polymer membraneis monodisperse and between about 0.5 and about 5.0 nanometers. The LLCpolymer porous membrane typically forms a coating on the porous support.

The composite membrane of the invention is useful for separationprocesses involving aqueous and nonaqueous solutions as well as gases.The composite membrane can also be made in flexible form, which allowsit to be used in a variety of membrane configurations (e.g.spiral-wound).

The present invention creates nanostructured porous polymer membranes inwhich the arrangement, size, and chemical properties of the pores may betailored on the molecular level by using polymerizable lyotropic (i.e.,amphiphilic) liquid crystals (LCs) as building blocks. These materialscan act as novel nanoporous membranes capable of selectively removingnanometer-size impurities, organic molecules, certain ions, and othercontaminants from solutions based solely on molecular size. In addition,the incorporation of chemical complexing agents in the nanopores ofthese materials can enable other forms of separation processes.

The invention also provides a method for making nanofiltration membraneswhich can be simpler than that for making currently availablenanofiltration membranes. In an embodiment, the invention provides amethod for making a composite nanofiltration membrane comprising thesteps of: providing a porous support, preparing a solution comprising aLLC monomer, an organic solvent for the monomer, and water, wherein theorganic solvent is selected to be compatible with the support; applyinga layer of the solution onto the porous support; evaporating the solventfrom the solution; and cross-linking the LLC monomer. In an embodiment,the layer of solution is applied to the porous support by rollercasting.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates cross-linking of an inverted hexagonal (H_(II)) LLCphase to form a polymer network containing hexagonally packedwater-filled nanopores.

FIG. 2 illustrates a bicontinuous cubic (Q) LLC phase.

FIG. 3 shows several H_(II) forming monomers.

FIG. 4 illustrates a monomer capable of forming L, H_(I) and Q_(II)phases.

FIG. 5 schematically illustrates the roller casting process.

FIG. 6A illustrates the percent dye rejected during flow of a dyesolution through a membrane of the invention for various cationic dyes.

FIG. 6B illustrates the percent dye rejected during flow of a dyesolution through a membrane of the invention for various anionic dyes.

DETAILED DESCRIPTION OF THE INVENTION

In an embodiment, the invention provides a composite nanofiltrationmembrane comprising: a porous support; and a LLC polymer porous membraneattached to the support. As used herein, a “membrane” is a barrierseparating two fluids that allows transport between the fluids. A“fluid” may be a liquid or a gas. A “composite” membrane comprises amembrane combined with a porous support.

As used herein, “nanoporous” signifies a pore size between about 0.5 andabout 6 nm in diameter and a “nanofiltration membrane” has an effectivepore size between about 0.5 and about 6 nm. “Ultraporous” signifies apore size between about 2.5 and about 120 nm and an “ultrafiltrationmembrane” has an effective pore size between about 2.5 and about 120 nm.“Microporous” signifies a pore size between about 45 nm and about 2500nm and a “microfiltration membrane” has an effective pore size betweenabout 45 nm and about 2500 nm. The effective pore size of a membrane isthe pore size of the part of the membrane which performs most of theseparation function. For the composite nanofiltration membranes of theinvention, the LLC porous membrane is a nanofiltration membrane whilethe porous support has a larger average pore size. In an embodiment, theLLC porous membrane has a pore size between about 0.5 and 5.0 nm. In anembodiment, the porous support is ultraporous.

As used herein, a “LLC polymer” is composed of polymerized lyotropicliquid crystal monomers in an ordered assembly. As used herein, “LLCmonomers” are polymerizable amphiphilic molecules that spontaneouslyself-assemble into fluid, yet highly ordered matrices with regulargeometries of nanometer scale dimension. LLC mesogens are amphiphilicmolecules containing one or more hydrophobic organic tails and ahydrophilic headgroup.

LLC monomers useful for the present invention are those that form an LCphase in the presence of water that contains ordered, monodisperse,aqueous nanopores, and those that can be polymerized into a cross-linkednetwork with substantial retention of the original LC phasemicrostructure. As used herein, “nanometer scale dimension” refers topore dimensions between about 0.5 and about 5 nm. LLC monomers usefulfor the present invention can form aqueous nanopores having a diameterbetween about 0.5 and about 5 nm. As used herein, a “monodisperse” poresize has a variation in pore size from one pore to another of less thanca 15% (specifically an ideally narrow Poisson distribution). For poresformed by some LC phases (e.g. bicontinuous cubic phases), the pore sizeof a given pore will vary along the pore channel. For pores whosedimensions vary along the pore channel, a comparison of pore sizes ismade at equivalent positions along the channel.

Polymerizable LLCs (i.e., cross-linkable surfactants) have been designedthat spontaneously form the inverted hexagonal (H_(II)) LLC phase in thepresence of a small amount of water. FIG. 1 schematically illustratesaqueous nanopores (5) and organic domains (7) in the H_(II) LLC phase.Upon photopolymerization or photo-cross-linking, robust polymer networks(17) containing hexagonally packed, extended water channels (15) withmonodisperse diameters of nanometer-scale dimensions are produced (FIG.1). The network has a pore structure of hexagonally ordered, cylindricalnanopores. The pore structure is substantially determined or controlledby the inverted hexagonal phase which was formed by the monomers. Thereis typically some contraction of the structure, approximately 5 vol %,on heavy cross-linking of the polymer into a network. A slight decreasein x-ray diffraction spacings was seen in the H_(II) phase afterpolymerization. If formed into a polymer film, these networks can beused as membranes. The resulting membranes may contain polydomains.

Different porous architectures can be achieved via the use of other LLCmonomers that form different mesophases. For example, polymerizable LLCphases with bicontinuous cubic (Q) architectures have interconnected 3-Dnanochannels (FIG. 2). In this case, the polymerized network has a porestructure of interconnected, ordered 3-D nanopores. The pore structureis substantially determined or controlled by the bicontinuous cubicphase which was formed by the monomers.

Several polymerizable LLCs (i.e., cross-linkable surfactants) have beendesigned that spontaneously form the inverted hexagonal (H_(II)) LCphase in the presence of a small amount of water (Gin et al. (2001) Acc.Chem. Res. 34, 973-980; U.S. Pat. No. 5,849,215; WO 98/30318). Withoutwishing to be bound by any particular theory, it is believed thatamphiphiles with a tapered shape (i.e. a small hydrophilic headgroup anda broad flattened hydrophobic tail section) should prefer to pack toform the H_(II) phase. In one embodiment, monomer 1 is used as thepolymerizable LLC. Monomer 1 forms a cross-linked H_(II) phase withhexagonally ordered, cylindrical water pores that are approximately 1.2nm in diameter and monodisperse in size. A synthesis method for monomer1 is disclosed in U.S. Pat. No. 5,849,215 and WO 98/30318. Other LLCmonomers that form the H_(II) phase and/or derivatives of monomer areshown in FIG. 3 (Gin et al. (2001) Acc. Chem. Res. 34, 973-980). Itshould be noted that these monomers can be made with different taillengths and head groups, which alter the nanopore dimensions. Also,mixtures of the aforementioned LLC monomers can also be produced totailor pore dimensions (Smith, R. C., Ph.D. Thesis (1999), University ofCalifornia, Berkeley).

Polymerizable LLCs have also been designed that spontaneously form thebicontinuous cubic (Q) LC phase. These mesogens include Geminisurfactant monomers. Monomer 2 forms a bicontinuous cubic phase(Pindzola, B. A., Ph.D. Thesis (2001), University of California,Berkeley). A single tailed version of monomer 2 exhibits similarbehavior but requires added cross-linker to form a cubic network uponphotopolymerization (Pindzola, B. A.; Hoag, B. P.; Gin, D. L. J. Am.Chem. Soc. 2001, 123 (19), 4617-4618.) (FIG. 4).

The pore size of the nanoporous LLC assemblies can be tuned viamodification of the parent LC monomer. (Resel, R; Leising, G.; Markart,P.; Kreichbaum, M.; Smith, R.; Gin, D. “Structural Properties ofPolymerised Lyotropic Liquid Crystal Phases of3,4,5-Tris(ω-acryloxyalkoxy)benzoate Salts,” Macromol. Chem. Phys. 2000,201 (11), 1128). It is believed that the pore size for H_(II) phases canbe tuned within the range 0.5-2.0 nm. It is believed that the pore sizefor bicontinuous cubic phases can extend up to 5 nm. Pore size and porearchitecture may also be tuned by changing temperature, pressure, andmixture composition, since LLC phase behavior is known to depend on allthree parameters.

The LLC polymer porous membrane is attached to the porous support. In apreferred embodiment, the LC membrane is formed in situ as a coating onat least a part of the surface of the porous support and the attachmentbetween the membrane and the support is made during the membranefabrication process. The thickness of the membrane can be measured asthe coating thickness. In situ formation of the LLC membrane ispreferred for membranes thin enough that the membranes would be expectedto tear during transfer of the membrane from one membrane to another. Itis expected that the membrane thicknesses of interest for the presentinvention would be susceptible to tearing when peeled from a substrate.Without wishing to be bound by any particular theory, the attachmentbetween the LLC membrane and the support during in situ fabrication mayresult from mechanical interlocking of the LLC membrane and the supportdue to penetration of the LLC material into the pores of the supportduring fabrication of the LLC membrane.

The porous support membrane gives physical strength to the compositestructure. The porous support is selected to be robust enough towithstand the pressure differential applied across it during filtration,typically up to 100 psi (0.6895 MPa). The support should also bethermally stable over approximately the same temperature range as theLLC membranes to be used.

The support is also selected so that it is permeable to the liquid orgas to be filtered. Preferably, the porous support membrane has asmaller flow resistance than the LLC membrane. The surface pore sizeshould be sufficiently small and the support surface sufficiently smooththat the LLC polymer can form a continuous coating. The porous supportin this system is selected so that the diameter of the pores is lessthan about 10 microns. The preferred pore size of the support may dependon the composition of the casting solution. In an embodiment, the poroussupport is an ultraporous membrane. In another embodiment, the supporthas a pore size less than about 0.1 micron.

The support is selected to be compatible with the solution used for LCmembrane formation, as well as to be compatible with the liquid or gasto be filtered. When the solution used for LC membrane fabrication andthe support are compatible, the support is resistant to swelling anddegradation by the solution used to cast the LC polymer porous membrane.In particular, the organic solvent used in the solution and the supportare selected to be compatible so that the support is substantiallyresistant to swelling and degradation by the organic solvent. Swellingand/or degradation of the support by the solvent can lead to changes inthe pore structure of the support. If the membrane is to be used forwater based separations, the porous support is sufficiently hydrophilicfor water permeation.

The porous support may be made of any suitable material known to thoseskilled in the art including polymers, metals, and ceramics. In variousembodiments, the porous support is a polyacrylonitrile (PAN),polyacrylonitrile-co-polyacrylate, polyacrylonitrile-co-methylacrylate,polysulfone (PSf), Nylon 6, 6, poly(vinylidene difluoride), orpolycarbonate support. The support may also be an inorganic support suchas a nanoporous alumina disc (Anopore™ Whatman, Ann Arbor, Mich.). Theporous support may also be a composite membrane.

For the H_(II) films, the flux through the LC polymer membrane isimproved when the nanopores are open, aligned and continuous throughoutthe LC polymer film. For polydomain, cross-linked H_(II) films, not allof the nanopores are necessarily open, aligned, and continuousthroughout the film. The alignment of the nanopores is generallyimproved by decreasing the LC polymer membrane thickness. Additionalalignment techniques known to those skilled in the art which arecompatible with the LC polymer membrane processing techniques describedherein may also be employed, for example application of external forces(for example, shear forces), interactions with surfaces, andapplications of large magnetic or electric fields. For the H_(II) LCpolymer membranes used in the present invention, it is preferred thatthe membrane thickness be less than about 10 microns. In differentembodiments, the H_(II) LC polymer membrane thickness may be less thanabout 5 microns, less than about 2 microns, less than about 1 micron, orless than about 0.5 microns. In different embodiments, the Q LC polymermembrane thickness may be less than about 10 microns, less than about 5microns, less than about 2 microns, less than about 1 micron or lessthan about 0.5 microns.

The flux rate through the composite membrane as a whole depends upon thepressure differential applied across the membrane as well as on thepermeability of the LLC polymer membrane. The composite membranes of theinvention are capable of sustaining pressure differences of greater than50 psi (0.3447 MPa) and obtaining aqueous solution flux rates greaterthan about 0.2 kg/m² hour. It is believed that unsupported LLC membraneswith thicknesses less than about 100 microns would be unable towithstand this pressure differential.

Furthermore, the LC polymer membrane can be fabricated with chemicalcomplexing agents in the nanopores. These chemical complexing agents maybe inorganic or organic entities that have the ability to interactreversibly or irreversibly with various solutes that enter the membrane.These chemical complexing agents may include, but are not limited to,metal ions such as Cu⁺, Cu²⁺, Ag⁺, Co²⁺, Sc³⁺, and aminefunctionalities.

A variety of methods can be used to prepare the composite nanofiltrationmembrane comprising a porous LC polymer membrane on a porous support.One method for making the composite nanofiltration membranes of theinvention comprises the steps of: providing a porous support; preparinga solution comprising a LLC monomer, an organic solvent for the monomer,and water; applying a layer of the solution onto a porous support;evaporating the solvent from the solution; and cross-linking the LLCmonomer. If desired, more than one layer of solution may be applied tothe support to form multiple layers of porous LC polymer and therebycontrol the film thickness.

The solution used for applying the lyotropic LLC monomer, also known asthe “casting solution”, comprises a lyotropic LLC monomer, an organicsolvent, water, a polymerization initiator that can be photolytically orthermally activated, and an optional cross-linking agent. The solventmay be any low boiling point solvent that dissolves the monomer. Amixture of one or more solvents may also be used. Useful solventsinclude, but are not limited to, methanol and diethyl ether. Thecross-linking agent is not required if the monomer can cross-linkwithout a cross-linking agent. In one embodiment, the monomer isdissolved in the organic solvent, and then the water and the optionalcross-linking agent are added. For monomer 1, casting solutions include,but are not limited to, solutions of approximately 5-11% monomer, 2-3%water, 0.5-1% 2-hydroxy-2-methylpropiophenone, balance methanol.

Application of the solution to the support can be achieved by anysolution based process known to the art, including painting, rolling,spraying, and inkjet printing of the solution onto the support. Thesolution is applied to form a coating on at least a portion of thesurface of the support. It is preferred that that coating be free ofsurface defects such as pinholes and scratches. In one embodiment, acommercial foam painting sponge or other such applicator can be used toapply the solution to the support. In another embodiment, the solutioncan be applied by roller casting. The amount of material on the supportcan be controlled by the number of applications and the concentration ofthe casting solution. For a given casting solution, a single applicationcan produce a lyotropic liquid crystal film with better pore alignmentthan multiple applications. It is believed that some of the solutionpenetrates into the support, with the extent of penetration depending onthe nature of the solution, the support, and the application process.The penetration of the solution into the support is believed to helpattach the cross-linked LLC polymer film to the support.

The solvent may be evaporated from the film by allowing the solvent toevaporate at ambient temperature. Other temperatures and controlledatmospheres as known by those skilled in the art can be used toevaporate the solvent from the film.

The LLC phase can be photo-cross-linked by putting it under UV light inair or nitrogen at ambient temperature. Other temperatures as known bythose skilled in the art may be used during the cross-linking process.Other methods of cross-linking as known to those skilled in the art mayalso be used. For example, thermal cross-linking may be performed usinga cationic initiator as a cross-linking agent.

In an embodiment, the invention provides a process for separating acomponent of a first fluid mixture, the process comprising the steps of:

-   -   bringing said first fluid mixture into contact with the inlet        side of a separation membrane, said separation membrane        comprising a LLC polymer porous membrane attached to a        ultraporous support membrane,    -   applying a pressure difference across said separation membrane;        and    -   withdrawing from the outlet side of said separation membrane a        second fluid mixture wherein the proportion of said component is        depleted, compared with said first fluid mixture.

Components which can be separated from a fluid mixture using themembranes of the invention include organic molecules, ions, gases,impurities and other contaminants.

The invention provides methods of size-selective filtration of solutionsusing the composite membrane of the invention. One or more componentssuch as nanometer-size impurities, organic molecules, certain ions, andother contaminants can be removed from solution by selecting the porediameter of the LLC membrane to be smaller than the molecular size ofthe component(s) of interest.

Furthermore, the invention provides methods for other forms ofseparation processes. If a chemical complexing agent is incorporatedinto the nanopores of the composite membrane of the invention, thechemical complexing agent can interact reversibly or irreversibly withvarious solutes that enter the membrane. For example, if metal ions suchas Cu⁺, Cu²⁺, and Ag⁺ are incorporated into the nanopores, enhancedoxygen separation or separation of olefins from paraffins can beenabled. Amine functionalities would enable enhanced CO₂ separation fromother gases. Similarly, the incorporation of water-stable catalyticentities in the nanopores of these materials may also offer the optionof catalytically degrading organic waterborne contaminants into morebiodegradable forms during the nanofiltration process. The incorporationof chemical complexing agents into LLCs is known to the art (Gu, W.;Zhou, W.-J.; Gin, D. L. “A Nanostructured, Scandium-Containing Polymerfor Heterogeneous Lewis Acid Catalysis in Water,” Chem. Mater. 2001, 13(6), 1949-1951.; Gray, D. H.; Gin, D. L. “Polymerizable Lyotropic LiquidCrystals Containing Transition-Metal Ions as Building Blocks forNanostructured Polymers and Composites,” Chem. Mater. 1998, 10 (7),1827-1832.; Deng, H.; Gin, D. L.; Smith, R. C. “Polymerizable LyotropicLiquid Crystals Containing Transition-Metal and Lanthanide Ions:Architectural Control and Introduction of New Properties intoNanostructured Polymers,” J. Am. Chem. Soc. 1998, 120 (14), 3522-3523).

EXAMPLES Example 1 Fabrication of a Composite Nanofiltration Membrane byPainting

Monomer 1 was dissolved in methanol then water and a commercial radicalphoto-initiator was added to form a dilute casting solution. Theconcentration of components in the solution is listed in Table 1.2-hydroxy-2-methylpropiophenone is a photo-initiator. A commercial foampainting sponge was used to evenly apply the solution on apoly(acrylonitrile-co-methylacrylate), 6% methyl acrylate, PAN support.The support was manufactured by Membrane Technology and Research (MenloPark, Calif.). The pore size of this support is less than about 0.1micron. At the support surface the pore sizes typically ranges betweenabout 10 and about 50 nm.

After allowing the methanol to evaporate at ambient temperature, the LCmonomer coating was photo-cross-linked by putting it under UV light (365nm, ca. 1200 μW/cm²) in air for 2 h at ambient temperature.

TABLE 1 Component of solution (wt %) Source Grade/purity Monomer 1:9.53% Synthesized pure by NMR Methanol: 87.13% Aldrich HPLC grade Water:2.63% de-ionized 2-hydroxy-2-methylpropiophenone: 0.71% Aldrich 97%

The amount of material on the support was controlled by the number ofapplications and the concentration of the casting solution. It was foundthat the thinner the nanoporous LC top layer, the better the observedflux through the composite membrane. The flexible supported membrane wasthen cut to size for testing in commercial filtration cells.

X-ray diffraction (XRD) analysis of the coated film samples showed thatthe characteristic low-angle XRD peaks for the H_(II) phase were seen ontop of the peaks from the semi-ordered PAN support material. The coatedmembrane was folded five times and placed in the XRD beam in order toget sufficient diffraction intensity. TEM imaging and XRD structuralanalysis estimates the nanopore size as approximately 1.2 nm.

Example 2 Fabrication of a Composite Nanofiltration Membrane byRoller-Casting

A dilute casting solution was formed as in Example 1. The support wascommercial ultraporous polysulfone (PSf) (Hydranautics, P-100,Oceanside, Calif.). The pore size of this support was less than about0.1 micron.

The roller casting process is schematically shown in FIG. 5. The support(20) was placed on a flat substrate (not shown). As shown in the upperpart of FIG. 5, the solution (30) was placed at one edge of the support.A syringe can be used to control placement of the solution. As shown inthe lower part of FIG. 5, an applicator rod (40) was then rolled overthe support to distribute the solution. Roller guides were placed at theedges of the support to control the height of the roller above thesupport. The guides may be made of adhesive tape or of any suitablematerial known to the art. The roller may be a wire-wound stainless rod,a glass rod, or similar roller type applicator.

After allowing the methanol to evaporate at ambient temperature, the LCmonomer coating was photo-cross-linked by putting it under UV light (365nm, ca. 1200 μW/cm²) in a nitrogen flushed acrylic glove box anywherefrom 2 hours to overnight at ambient temperature. In particular, tominimize O₂ inhibition of the radical photopolymerization processespecially for sub-micron coatings, the LLC coated membranes are placedin a sealed large area photopolymerization cell, evacuated with dynamicvacuum to remove ambient and dissolved O₂, and then flushed with N₂during photolysis. This procedure allows degrees of polymerization ofgreater than 80% to be achieved reproducibly with submicron coatedfilms.

Using this procedure, uniform thin (approximately 0.3 micron) coatingswere obtained over a 6 by 9 cm area.

Example 3 Size Selective Filtration Using a Composite NanofiltrationMembrane

The composite membrane comprising a layer of LLC polymer (thickness lessthan 1 micron, approximately 0.3 micron) on an ultraporous PAN supportdescribed in Example 1 was cut to size and placed in a commercial 25 mmdiameter dead-end filtration cell (Advantec UHP25). Upon application ofan applied pressure of ca. 60 psi (0.4137 MPa) of nitrogen gas, watersolution fluxes on the order of 0.263-0.789 kg/m² hour were achievedthrough the membrane. For 3-layer approximately 1 micron films, thewater solution fluxes were approximately half that of the one layerfilms.

Using this configuration, highly selective molecular size filtration ofwater-soluble molecules in water has been demonstrated. When a watersolution of a blue dye (Alcian Blue pyridine variant) with a moleculardiameter (2.0 nm) slightly larger than the nanopores was passed throughthe membrane, the blue dye was completely filtered out. No trace of theblue dye was present in the filtrate solution, as confirmed byUV-visible spectroscopy. When a water solution of a red dye (phenolsulfone phthalein) with a diameter (0.96 nm) much smaller than thenanopores was passed through the membrane, the red dye was able to passthrough. Finally, when a 1/1 water mixture of the large blue dye and thesmall red dye (green color solution) was forced through the LC-coatedmembrane, a red solution was produced via filtration, indicatingmolecular size selectivity. Spectroscopic analysis of the filteredsolution only showed the presence of the red dye, indicating completerejection of the large blue dye

Additional nanofiltration experiments with cationic, rigid dyes ofdifferent molecular sizes, such as Methylene Blue, Brilliant Green, andEthidium Red, have subsequently revealed that the effective diameter ofthe nanopores is ca. 1.2-1.4 nm. FIGS. 6A and 6B show plots of singledye rejection as a function of dye size and charge. The experiments inFIGS. 6A and 6B shows that molecular sieving with these membranes is ageneral phenomenon that works for both cationic and anionic molecules.Furthermore, the uncoated support was unable to achieve the selectivityof the composite LLC membrane.

Those of ordinary skill in the art will appreciate that materials andmethods other than those specifically described herein can be employedin the practice of this invention without departing from the scope ofthis invention.

All references cited herein are hereby incorporated by reference intheir entirety to the extent that they are not inconsistent with thedisclosure herein.

1. A composite membrane comprising: a porous support; and a lyotropicliquid crystal (LLC) polymer porous membrane attached to the support,the LLC membrane having a thickness less than about 1 micron and a porestructure having hexagonally ordered, cylindrical nanopores.
 2. Thecomposite membrane of claim 1, wherein the pore size of the support isless than about 0.1 micron.
 3. The composite membrane of claim 1,wherein the thickness of the LLC membrane is less than about 0.5microns.
 4. The composite membrane of claim 1, wherein the LLC membraneis a single layer.
 5. The composite membrane of claim 1, wherein theporous support is selected from the group consisting ofpolyacrylonitrile (PAN), polyacrylonitrile-co-polyacrylate,polyacrylonitrile-co-methylacrylate, polysulfone (PSf), Nylon 6, 6,poly(vinylidene difluoride), and polycarbonate supports.
 6. Thecomposite membrane of claim 1 wherein the composite membrane has a fluxof an aqueous solution of at least 0.2 kg/(m² hr) with a pressuredifferential of 60 psi (0.4137 MPa).
 7. The composite membrane of claim1 wherein the pores of said LLC polymer membrane incorporate a chemicalcomplexing agent.
 8. The membrane of claim 1, wherein the LLC polymer isformed of polymerized LLC monomers in an ordered assembly and optionallya crosslinking agent and the LLC monomers form the inverted hexagonalphase.
 9. The membrane of claim 1, wherein the LLC polymer penetratesinto the pores of the support.
 10. A composite membrane comprising: aporous support; and a lyotropic liquid crystal (LLC) polymer porousmembrane attached to the support, the LLC membrane having a thicknessless than about 10 microns and a pore structure of interconnected,ordered, 3-D nanopores.
 11. The composite membrane of claim 10 whereinthe pore size of the support is less than about 0.1 micron.
 12. Themembrane of claim 10, wherein the porous support is selected from thegroup consisting of polyacrylonitrile (PAN),polyacrylonitrile-co-polyacrylate, polyacrylonitrile-co-methylacrylate,polysulfone (PSf), Nylon 6, 6, poly(vinylidene difluoride), andpolycarbonate supports.
 13. The membrane of claim 10, wherein the poresof said LLC polymer membrane incorporate a chemical complexing agent.14. The membrane of claim 10, wherein the LLC polymer is formed ofpolymerized LLC monomers in an ordered assembly and optionally acrosslinking agent and the LLC monomers form a bicontinuous cubic phase.15. The membrane of claim 10, wherein the LLC polymer penetrates intothe pores of the support.
 16. A method for making a compositenanofiltration membrane comprising an ultraporous support and a LLCpolymer porous membrane attached to the support, the method comprisingthe steps of: providing the ultraporous support; preparing a solutioncomprising a LLC monomer, an organic solvent for the monomer, apolymerization initiator and water, wherein the organic solvent isselected to be compatible with the support; applying a layer of thesolution onto the support; evaporating the solvent from the solution;and cross-linking the LLC monomer.
 17. The method of claim 16 whereinthe solution is applied to the support by roller casting.
 18. The methodof claim 16 wherein the pore size of the support is less than about 0.1micron.
 19. The method of claim 16 wherein the LLC monomer is selectedfrom the group consisting of inverted hexagonal (H_(II)) formingmonomers and bicontinuous cubic (Q) forming monomers.
 20. The method ofclaim 16 wherein the LLC monomer forms the inverted hexagonal phase. 21.The method of claim 20 wherein the LLC polymer membrane thickness isless than about 1 micron.
 22. The method of claim 20 wherein the LLCpolymer membrane thickness is less than about 0.5 microns.
 23. Themethod of claim 16 wherein the LLC monomer forms the bicontinuous cubicphase.
 24. The method of claim 23 wherein the LLC polymer membranethickness is less than about 10 microns.
 25. The method of claim 16,wherein the porous support is selected from the group consisting ofpolyacrylonitrile (PAN), polyacrylonitrile-co-polyacrylate,polyacrylonitrile-co-methylacrylate, polysulfone (PSf), Nylon 6, 6,poly(vinylidene difluoride), and polycarbonate supports.
 26. A methodfor making a composite nanofiltration membrane comprising a poroussupport and a LLC polymer porous membrane attached to the support, themethod comprising the steps of: providing the porous support; preparinga solution comprising a LLC monomer, an organic solvent for the monomer,a polymerization initiator, and water, wherein the monomer forms theinverted hexagonal phase and the organic solvent is selected to becompatible with the support; applying a layer of the solution onto thesupport; evaporating the solvent from the solution; and cross-linkingthe LLC monomer, whereby the thickness of the LLC polymer porousmembrane is less than about 1 micron.
 27. A method for making acomposite nanofiltration membrane comprising a support and a LLC polymerporous membrane attached to the support, the method comprising the stepsof: providing a porous support; preparing a solution comprising a LLCmonomer, an organic solvent for the monomer, a polymerization initiator,and water, wherein the monomer forms the bicontinuous cubic phase andthe organic solvent is selected to be compatible with the support;applying a layer of the solution onto the support; evaporating thesolvent from the solution; and cross-linking the LLC monomer, wherebythe thickness of the LLC polymer porous membrane is less than about 10microns.
 28. A process for separating a component of a first fluidmixture, comprising the steps of: bringing said first fluid mixture intocontact with the inlet side of a separation membrane, said separationmembrane comprising a LLC polymer porous membrane attached to aultraporous support membrane, wherein the pore structure of the LLCpolymer porous membrane has hexagonally ordered, cylindrical nanoporesor interconnected, ordered, 3-D nanopores, applying a pressuredifference across said separation membrane; and withdrawing from theoutlet side of said separation membrane a second fluid mixture, whereinthe proportion of said component is depleted, compared with said firstfluid mixture.
 29. The process of claim 28, wherein the pores of saidLLC polymer membrane are smaller than the molecular size of saidcomponent.
 30. The process of claim 28, wherein the pores of said LLCpolymer membrane incorporate a chemical complexing agent.